Organic polysulphides, which can be represented by the general formula RS.sub.n R' in which n is an integer ranging from 2 to 15 and R and R' are optionally substituted hydrocarbyl radicals, are used in numerous applications such as, for example, presulphurization of hydrorefining catalysts and addition to elastomers. They are also excellent extreme-pressure and anti-wear additives for lubricants, and are therefore involved in the composition of lubricating formulations for gearboxes and for machining metals. Most applications require a polysulphide which is odorless, precipitate-free, clear and stable in the long-term.
However, organic polysulphides are generally prepared by reacting a mercaptan with elementary sulphur in the presence of a basic catalyst; they may also be prepared from an olefin, sulphur and/or hydrogen sulphide, optionally in the presence of a basic catalyst. Products prepared in this way contain or release hydrogen sulphide and/or mercaptans, which are difficult to remove.
Hydrogen sulphide is toxic and, together with mercaptans, causes unpleasant odours. Furthermore in the possible presence of catalytic residues, the residual mercaptan and hydrogen sulphide make the finished product unstable, leading to some degree of turbidity or the appearance of precipitates in the course of time.
In order to deodorize and stabilize organic polysulphides, it is essential to remove the mercaptan and hydrogen sulphide residues by suitable treatment. Various processes for solving this problem have already been described:
washing the raw polysulphide with an acid and/or babe solution (U.S. Pat. No. 5,155,275);
washing the raw polysulphide with an oxidizing solution in the presence of a catalytic amount of base (U.S. Pat. No. 5,206,439);
washing the raw polysulphide with an alcoholic or aqueous solution of a metal salt (U.S. Pat. No. 5,403,961);
treating the raw polysulphide with an acid and subsequent distillation (U.S. Pat. No. 5,530,163);
treating the raw polysulphide with an alkene oxide in the presence of a catalytic amount of base (Patents JP 58-140063 and U.S. Pat. No. 5,218,147).
These processes are not entirely satisfactory. The washing processes require the addition of solvents or aqueous solutions which are difficult to separate from the polysulphide at the end of the treatment, unless time-consuming and expensive separation means are employed. The process involving final distillation is expensive in terms of energy and is applicable only to light polysulphides. Alkene oxides are toxic and carcinogenic.
Some methods do not make it possible, in a single treatment, to reduce the residual mercaptan sulphur level to a level which is low enough to stabilize the polysulphide; the need to carry out several successive treatments makes these time-consuming and expensive methods.